Method and device for filtering yeast and other plant cell material on rotary vacuum filter

ABSTRACT

A method and device for filtering a suspension (14), for example of yeast or other plant cell material, on a rotary vacuum filter (10), beneath which a trough (4) containing the suspension (14) is located. In order to improve the balance between dipping zone and drying zone, according to the invention a take-off knife (1) for taking off the filter cake (8) resulting from the filtering is attached to the trough (4). The level of the suspension (14) is maintained constant, and the trough (4) is moved by means of a feeding device (7) to the center of the drum (12) of the vacuum filter (10) while the drum is rotating.

This invention relates to a method and device for use at the filteringof a suspension, for example bakery yeast mixed with common salt (NaCl)or other active electrolyte according to the cytorrhysis method or atthe filtering of other plant cell material with or without salt.

At such method, first the rotary drum filter is pre-coated with filterauxiliary, so-called precoat, for example starch. The filtering of, forexample, the yeast suspension mixed with electrolyte is carried out froma trough located below the lowermost point of the rotary filter drum.The washing of the salt out of the filter cake is effected in knownmanner and the filter cake formed at the filtering process is taken offby a take-off knife.

Examples of the known art in the field with respect to filters hereconcerned are the so-called Lundin filter or the so-called rapid filterwith bottom trough, which are marketed by AB Alfa Laval, Sweden.

Very small changes in the concentration, composition or properties of aproduct require the most varying contact times between filter surfaceand suspension in order to obtain a filtering result which is optimum.For this reason, the filtering results with the present filters rarelyare at optimum, because the filters in most cases are adjusted atoptimum to only one product. The required variation in contact timebetween the suspension and the active filter surface can vary between adipping angle of 180°, i.e. where half the drum is dipped into thesuspension, for example at the filtering of must in wine-cellars, and adipping angle of 1°-2°, for example at the filtering of sediments inyeast in wine-cellars and breweries. This implies that the ratio betweendipping zone and drying zone in the first-mentioned case amounts to 1:1and in the latter case to the magnitude 1:250.

When the suspension level in the trough at conventional bottom troughfilters is lowered in order to reduce the contact time, and the take-offknife remains in its original position, a corresponding part of theactive filter surface is lost and simultaneously arises there airleakage. The result, this, is reduced filtering capacity. When, instead,the level in the trough is raised, this yields optimum filteringcapacity for a very limited number of suspensions. For the majority ofsuspensions, for example in the ferment industry, the resulting longcontact time between filter surface and suspension cannot be utilizedfor cake formation. The result is also in this case a substantiallyreduced filtering capacity.

Most of the existing rotary vacuum filters, thus, show the weakness thatit is difficult to achieve good balance between coating zone and dryingzone, i.e. between the drum portion where cake formation takes place,and the drum portion where the filter cake achieves by degrees maximumdry content.

A problem associated therewith is, that at a trough adjusted to acertain suspension said balance is changed during the filtering process,because the knife is moved relative to the trough and the suspensionlevel.

The present invention solves this problem and offers advantages also inother respects, as will appear from below. 0009he present invention,thus, relates to a method at the filtering of a suspension, for exampleyeast or other plant cell material on a rotary vacuum filter, beneathwhich a trough containing the suspension is located. The invention ischaracterized in that a take-off knife for the filter cake produced bythe filtering process is attached to the trough, and the suspensionlevel in the trough is held constant, and that at least the troughportion carrying the take-off knife is moved by a feeding device to thecentre of the filter drum while the drum is being rotated.

The invention, further, relates to a device of the kind and having themain characterizing features as defined in claim 5.

The invention is described in greater detail in the following, withreference to the accompanying drawings, in which

FIG. 1 shows schematically, partially by vertical section, a devicewhere the invention according to a first embodiment thereof is applied,

FIG. 2 is a vertical section corresponding to the one shown in FIG. 1according to a second embodiment of the invention.

In FIG. 1 a rotary vacuum filter 10 is shown capable to rotate in thedirection indicated by the arrow 11. Beneath the drum 12 a trough 4 islocated. This is a conventional design of a rotary vacuum filter. At theforward end 3 of the trough 4, according to a first embodiment, atake-off knife 1 is ridigly attached to a holder 2 at the trough 4.Preferably at the rear end of the trough an equalizing doctor 6 isridigly attached, the object of which is control of the thickness of thefilter cake 8 in order to obtain efficient washing-out of salt, which issupplied, for example, in a yeast concentrate.

The equalizing doctor is of an elastic material, for example rubber,Vulcan®, Tricolan®.

The trough is supported on a feeding device 7 comprising hydraulic orpneumatic cylinders 13 or screw spindles or corresponding lifting means.

The trough 4 occupies from its forward to its rearward edge normally anangle of about 40°-60° of the circumference of the filter drum inFIG. 1. The trough, however, may be designed greater or smaller foryielding greater or smaller dipping angles, i.e. the angles at whichcontact prevails between filter surface and suspension.

According to a first embodiment of the present invention, thus, theforward edge 3 of the trough 4 is equipped with a take-off knife 1 forthe filter cake 8. The knife constitutes an inherent part of the trough.

The feeding device 7 is capable during the filtering process tocontinuously move the trough 4, which contains the suspension 14 mixedwith electrolyte, in the direction to the drum centre, i.e. straightupward in FIG. 1.

Simultaneously with the movement of the trough 4, the take-off knife 1rididly attached in the trough 4 for taking off the filtered cake 15and, when provided, the equalizing doctor 6 are moved upward through thepre-coat layer, which thereby during the rotation of the drum 12 isde-scaled in a thin layer together with the filter cake. The liquidlevel in the trough is maintained constant, preferably by a spillway 5.

Said level is maintained a short distance below the level of thetake-off knife, as appears from FIG. 1.

Owing to the constant level of the suspension in the trough, togetherwith the upward movement of the trough, the dipping angle remainsconstant during the upward movement of the trough, irrespective of thethickness of the pre-coat layer.

The suspension, thus, is supplied immediately behind the knife 1. Thefilter drum 12 is hereby coated with filter cake 8 about all of itscircumference, except for the short portion thereof which is locatedbetween the knife 1 and the surface 9 of the suspension. This shortportion generally should be about 20 to 50 mm.

At this embodiment the trough 4 is adjusted to optimum dipping anglewhich, thus, is constant during the entire filtering operation.

The filtering is hereby extremely efficient.

However, as mentioned above in the introductory part, the dipping anglevaries for different suspensions, so that different troughs must beavailable which render possible dipping angles from 180° to 1°-2°,depending on the suspension.

According to a second embodiment of the invention, which is intendedprimarily for small installations, in which small quantities ofsuspensions with different filtering properties are to be filtered, apossibility of rapidly changing the dipping angle with one and the sametrough is provided.

In FIG. 2 such a trough 16 is shown, the depth of which is sufficientfor rendering possible such maximum dipping angles which possibly may berequired. The trough 16 according to this embodiment is provided withone or more apertures or slits 17,18,21 in the trough bottom at thelevel, at which the dipping angles in question are obtained. In theslits 17,18,21 a take-off knife 19 is prepared to be located. FIG. 3shows by fully drawn lines the knife 19 located farther down on thetrough, and by dashed lines a knife location corresponding to that shownin FIG. 1 is indicated. Of course, only one take-off knife 19 at a timeis used, and cover means 20 or corresponding means are provided tosealingly close by suitable fastening members 22 the slits 17,21 inwhich the take-off knife 19 is not located.

The take-off knife 19, thus, is attached upright from the bottom of thetrough 4 when a dipping angle smaller than the maximum one is desired.

The trough also at this embodiment is supported on the feeding device 7shown in FIG. 1 which advances the trough upward to the drum centre.Irrespective of the location of the take-off knife 19, the suspensionlevel in the trough is maintained a short distance below the take-offknife 19.

For maintaining the suspension level constant, a spillway 25,26,27 canbe provided for each respective knife location.

One of said spillways 25,26,27 which corresponds to the knife positionin question is connected, while the remaining two spillways are closed.

The filter cake 23 taken off by the take-off knife 19 is dischargedthrough the aperture 18 in question immediately before the knife 19,seen in the direction of movement of the drum 12. The apertures 17,18,21, thus, have a size sufficient for such discharge. The take-off knife19, further, extends all along or substantially all along the axiallength of the drum 12 and is sealed by sealing means (not shown) to thesides of the trough 16, so that the suspension 14 is prevented frompassing the knife 19 to the right in FIG. 2.

The knife 19 is attached in a slit 18 by suitable fastening members 24,in or against which the knife 19 is fixed.

At the discharge of the scaled-off filter cake also parts of the precoatlayer follow along.

By this embodiment, thus, one and the same installation can easily beadjusted to a suspension in question, which is filtered with the dippingangle being constant, irrespective of the position in which the take-offknife is located.

According to both embodiments of the invention, the dipping angle isadjusted prior to the filtering operation, in that the distance of thetrough 4 to the drum is adjusted by the feeding device 7.

By means of the present invention, thus, good balance between coatingzone and drying zone is maintained during the entire filtering process,because the dipping angle remains constant.

A furher advantage provided by the present invention is the reduction ofpre-coat consumption, because substantially the entire circumference ofthe drum is utilized.

At the embodiment described above, the feeding device 7 can be designedso as to move the trough 4 in parallel upward or to move the troughwhile inclining it slightly. According to a modified embodiment,however, the trough 4 is mounted pivotally by means of a bearingarrangement 25, which is shown by dashed lines in FIG. 2 and locatedbetween the lower surface of the trough and the support. The feedingdevice, which comprises members corresponding to those described above,is located in a point along the trough spaced from the bearingarrangement. The bearing arrangement, for example, may be located at thelowermost and, respectively, central point of the trough, or to the leftthereof, in FIG. 2 and, respectively, FIG. 1. The feeding device herecomprises only its right-hand portion 7,13 in FIGS. 1 and 2.

When the trough above and in the claims is stated to be moved to thecentre of the drum, this refers, thus, at least to that trough portion,at which the take-off knife 1,19 is located. The movement, of course, isnot restricted to take place entirely radially to the drum centre.

The present invention, thus, in a simple and inexpensive way solves theproblem referred to above in the introductory part.

Several modifications of the invention, of course, can be imaginedwithout abandoning the idea of the invention. The attachment of thetake-off knife to or in the trough, for example, can be varied in waysobvious to the expert, and also the feeding device can be designed in away other than shown at the embodiments.

The invention, thus, is not to be regarded restricted to the embodimentsdescribed above, but can be varied within the scope of the attachedclaims.

We claim:
 1. A method for filtering a suspension, for example of yeastor other plant cell material, on a rotary vacuum filter (10), beneathwhich a trough (4) containing the suspension (14) is located,characterized in that, in this combination, a take-off knife (1,19) fordescaling the filter cake (8) resulting from the filtering together witha thin layer of a precoat layer is secured on the trough (4);maintaining the level (9) of the suspension (14) in the trough (4)constant a small distance below the take-off knife whereby only a smallperipheral portion of the drum is not used for dipping or drying, andmoving at least that portion of the trough (4) which carries thetake-off knife (1, 19) by means of a feeding device (7) toward thecentre of the drum (12) of the vacuum filter (10) during filtration andwhile the drum (12) is rotating, to thereby provide a successive thinlayer descaling of the precoat and obtain a constant dipping angleduring filtration.
 2. A method as defined in claim 1, characterized inthat the take-off knife (1,19) is secured ajacent one upper edge of thetrough (4) when a maximum dipping angle obtainable with the trough isdesired.
 3. A method as defined in claim 1, characterized in that thetake-off knife (19) is secured to project upward from a location closerto the bottom of the trough (4) when a dipping angle is desired which isless than said maximum dipping angle obtainable when the take-off knifeis secured adjacent an upper edge of the trough.
 4. A method as definedin claim 3, characterized in that the filter cake (23) taken off by thetake-off knife (19) is discharged through an aperture (17,18,21) locatedin the bottom of the trough (4) before the knife (19), seen in thedirection of movement of the rotating vacuum filter.
 5. A device forfiltering a suspension, for example of yeast or other plant cellmaterial, on a rotary vacuum filter (10) comprising a drum (12) andbeneath the same a trough (4) for the suspension and a take-off knife(1, 19) for descaling the filter cake resulting from the filteringtogether with a thin layer of a precoat layer, characterized in that thetake-off knife (1,19) is removably secured to the trough (4), a feedingdevice supports the trough (4) and is operable to move at least thatportion of the trough (4) which carries the take-off knife (1, 19) inthe direction toward the centre of the drum (12) during rotation of thedrum; and that means (5; 25-27) are provided in said trough formaintaining the suspension level, in the trough, constant duringmovement of the trough and only a small distance below the take-offknife whereby only a small peripheral portion of the drum is not usedfor dipping and drying.
 6. A device as defined in claim 5, characterizedin that the take-off knife (1,19) is secured adjacent one upper edge ofthe trough (4).
 7. A device as defined in claim 5, characterized bymeans defining a plurality of elongate apertures extending parallel tothe drum axis and located in spaced apart relationship from a locationadjacent an upper edge of the trough to a location adjacent the bottomof the trough; said take-off knife being selectively secured in any oneof said apertures dependent on a desired dipping angle; and cover meansprovided for selectively closing those apertures in said trough otherthan that aperture in which said take-off knife is secured.
 8. A deviceas defined in claim 7, characterized in that each said aperture is ofsuch a size that a filter cake, taken off by said take-off knife locatedin a selected aperture, is caused to be discharged through said selectedaperture.